In the Horse Ownership screen, there are four options available to you when you click Sell Share. In each instance, the new owner of a share in a horse must exist in People & Firms.
Transfer the share to another owner: Use this to transfer an entire stake/share in a horse to another owner
Split the share to multiple owners: Sell a share to two or more owners.
Sell some of the share to another owner: Sell a portion of an existing share to another owner
Sell this share, but I don't have another owner to assign it to (yet): This removes an owner based on the Sale date, however this percentage of ownership is not going to be immediately reallocated.
Changing horse ownership does not affect any invoices or owner statements already issued.
Whenever altering horse ownership, Ardex strongly recommends using the Validate button to ensure 100% ownership at all times. Failing to do this can affect customer invoicing.
For more information with horse ownership, see the Ardex Premier User Guide - Horse Ownership in Ardex - Selling Owner Shares (pg. 6).